Polyesters are widely used as extrusion and injection molding resins for the fabrication of various articles for household or industrial use, including appliance parts, containers and auto parts. A majority of the polyesters are composed of polyethylene terephthalate (PET) or materials where PET has been modified by the addition of other diols and/or dicarboxylic acid esters. The polyesters are usually prepared by reacting a dialkyl ester of a dicarboxylic acid and a diol, for example, dimethylterephthalate and ethylene glycol in the presence of a transesterification catalyst.
In the above mentioned polymerization reaction, trace amounts of dicarboxylic acids deactivate the transesterification catalysts resulting in undesirable prolonged transesterification times or no ester exchange. If zinc or manganese are used as the transesterification catalyst, the catalyst is rendered essentially ineffective. This severely limits the monomers which can be used to prepare a polyester. For example, in order to copolymerize a dicarboxylic acid with dimethylterephthalate, the dicarboxylic acid must be converted to its corresponding dicarboxylic acid ester. Therefore, it would be economically advantageous to be able to polymerize a mixture containing a dialkyl ester of a dicarboxylic acid and a dicarboxylic acid in the presence of standard transesterification catalysts.
The process of the present invention has eliminated the need to convert a dicarboxylic acid to its corresponding diester before copolymerizing it with a dimethyl ester of a carboxylic acid in the presence of standard transesterification catalysts.